1. Field of the Invention
This invention relates to a surface acoustic wave device and, more particularly, it relates to a surface acoustic wave device having a non-symmetrical frequency response characteristic with respect to the central frequency, for use, for example, in a video intermediate frequency circuit of a television receiver.
2. Description of the Prior Art
A typical surface acoustic wave filter has been put into practical use which comprises a transducer including a piezoelectric material, for example, piezoelectric ceramic of, for example, PZT, a single crystal of such as LiNbO.sub.3 or a piezoelectric thin film such as ZnO and input side and output side electrodes disposed on a surface of the piezoelectric material. At least one of the input side and output side electrodes takes the form of an interdigital electrode. The interdigital electrode comprises a set of comb-shaped electrodes, each comb-shaped electrode having a plurality of electrode fingers and a common electrode for commonly connecting those electrode fingers at one end thereof. The set of comb-shaped electrodes is disposed such that the plurality of electrode fingers of one of the comb-shaped electrodes lie in an interdigital manner with those of the other comb-shaped electrode.
Due to the fact that of the surface acoustic wave filter is small in size and requires no adjustment, it has been utilized in a wide range of equipment. Of late, such a surface acoustic wave filter has widely been used as filter in the video intermediate frequency circuit of television receivers. As is well known, a filter for use in the video intermediate frequency circuit of the television receiver must include a sound trap, etc. It is therefore necessary to provide a filter having a frequency response characteristic that is non-symmetrical with respect to the central frequency, i.e., a frequency f.sub.0 intermediate between the adjacent channel picture carrier frequency and the adjacent channel sound carrier frequency.
One conventional way to provide a surface acoustic wave filter having a non-symmetrical frequency response characteristic through the weighting technique is to change the overlapping length and the center-to-center distance between the two adjacent electrode fingers, i.e., the electrode pitch, in the propagating direction of the surface acoustic wave. Such an interdigital electrode may be referred to as a variable pitch type. More particularly, if a non-symmetrical frequency characteristic required for the video intermediate frequency filter, for example, is subjected to a Fourier reverse conversion, then an impulse response characteristic as depicted in FIG. 1 is provided. The resulting impulse response characteristic includes an imaginary part as a result of Fourier reverse conversion and the time period between the respective peak points of real part where the imaginary part is zero is unequal, because of the non-symmetry of the frequency response characteristic. With the design of the interdigital electrodes in association with the resulting impulse response characteristic, the surface acoustic wave filter is enable to manifest a desired frequency characteristic. This design may be achieved by proper selection of the overlapping length of the adjacent electrode fingers, i.e., the surface acoustic wave exciting and receiving region, and proper selection of the electrode pitch, the former being selected to be proportional to the amplitude of a respective one of peaks (as denoted by the arrows) in the impulse response characteristic and the latter being selected to be proportional to the period between the adjacent peak points in the impulse response characteristic. As is understood from FIG. 1, the surface acoustic wave filter thus obtained has the interdigital electrodes of unequal electrode pitches and thus is of a so-called variable pitch type because of non-uniformity of the time period between the respective peak points where the imaginary part is zero.
Although the conventional approach using the variable pitch type interdigital electrodes is successful in attaining a non-symmetrical frequency characteristic, it has the outstanding problems that electrode patterns are difficult to design due to unequal pitches of the electrodes and the electrodes are easily broken or short-circuited in high frequency applications due to mixture of wide and narrow electrode fingers.
Several approaches have been proposed in an attempt to overcome the above problems in attaining a non-symmetrical frequency characteristic with the aid of the interdigital electrodes of equal pitch. One of those approaches is an odd/even function method and another one is a mirror method or reflection method as will be discussed below.
The former method or the odd/even function method assumes that H.sub.2 (.omega.) meets a precondition H.sub.1 (.omega.-.omega..sub.0)=H.sub.2 (.omega..sub.0 -.omega.), wherein H.sub.1 (.omega.) is the linear representation of a desired frequency characteristic. The correlation between H.sub.1 (.omega.) and H.sub.2 (.omega.) is illustrated in FIGS. 2A and 2B. Assuming H.sub.R (.omega.) is the even component and H.sub.I (.omega.) is the odd component with the following definitions thereof, those components are correlated as shown in FIGS. 3A and 3B. ##EQU1##
From equations (1) and (2), H.sub.1 (.omega.) may be rewritten as follows: EQU H.sub.1 (.omega.)=H.sub.R (.omega.)-.sub.j H.sub.I (.omega.) (3)
The impulse response characteristic is a Fourier conversion of the above equation (3), with the following definition. ##EQU2##
The impulse response characteristics as defined by h.sub.R (t) and -.sub.j h.sub.I (t) in equation (4) are plotted with the solid line and the dotted line, respectively, in FIG. 4 where the sampling interval is t=1/2f.sub.0. A review of the two impulse response characteristic curves in FIG. 4 indicate that the peak-to-peak interval is 1/22f.sub.0, which is proportional to .lambda..sub.0 /2 in terms of wavelength, and the respective peaks in one of the curves align exactly with the center of the peak-to-peak intervals of the other curve. It is pointed out that the interdigital electrode corresponding to the impulse response characteristic as depicted by the solid line defines the even components and that the interdigital electrode corresponding to the impulse response characteristic as depicted by the dotted line defines the odd components.
An electrode pattern as shown in FIG. 6 is an electrode pattern formed by dividing the interdigital electrodes into two rows 2 and 3 in conformance to the two impulse response curves shown in FIG. 4 and electrically connecting the interdigital electrodes in parallel. This is fully disclosed in an article entitled "One Approach to Design Surface Acoustic Wave Filter" by Nakamura and Shimizu (172nd Acoustics Symposium, Electrical Communication Institute of Tohoku University issued on Sept. 28, 1972).
In FIG. 6, there is shown a set of interdigital electrodes 1 and 4. Electrode 1 is defined by two interdigital electrodes 2 and 3 disposed in a direction normal to the wave propagating direction, with electrode 3 excites and receives either the even or odd components and the electrode 2 excites and receives the other of the even or odd components. The electrode 4 is adapted to cover paths for waves between the two electrodes 2 and 3.
The electrode pattern of FIG. 6 is however disadvantageous in that the region for exciting and receiving the surface waves extend through a surface wave substrate and the substrate must be large because the electrode 1 includes the two interdigital electrodes which extend in a direction perpendicular to the propagating direction. Furthermore, the electrode pattern is undesirable from the viewpoint of a pattern layout due to the fact that the regions where the intensity of exciting and receiving the surface waves is high, i.e., overlapping length is long, are located on both side portions and the common electrode is located at the center of the electrode pattern.
One way to avoid the above problems is to combine the two impulse response characteristics into a single one as plotted in FIG. 5 and form an electrode pattern in conformance to such combined response characteristic as indicated in FIGS. 7A and 7B. This method assures a non-symmetrical frequency characteristic with the interdigital electrodes of the same pitch. Referring to FIGS. 7A and 7B, one of the interdigital electrodes 5 is formed by main electrode fingers 6, 7, 8 and 9 each having an electrode width of .lambda..sub.0 /8 and spaced at an electrode pitch of .lambda..sub.0 /4 the two adjacent main electrode fingers being connected to common portions held at different potentials, respectively, and having different lengths and auxiliary electrode fingers 10, 11, 12 and 13 each having an electrode with of .lambda..sub.0 /8 and spaced at an electrode pitch of .lambda..sub.0 /4 and furthermore facing against free ends of the respective main electrode fingers and being connected to the common portions held at the different potentials.
With the interdigital electrode design of FIG. 7, the even component is excited and received by a region where the adjacent main electrode fingers 7 and 8, which are at different potentials overlap with each other in the lengthwise direction of the electrode fingers (as defined by the right upward hatching regions), whereas the odd component is excited and received by a region where the main electrode fingers 6 and 9 and the adjacent auxiliary electrode fingers 11 and 12 cross with each other (as denoted by the right downward hatching regions), the odd component being excited and received with a distance deviation of .lambda./4 with respect to the even component. With such an interdigital electrode design, it is possible to reduce the length of the electrodes in a direction normal to the propagating direction of the surface waves and therefore minimize the dimension of the surface wave substrate. However, because the surface waves are excited and received at regions where the respective electrode fingers 6 and 7 cross with the electrode fingers 8 and 9, respectively, (as defined by the cross hatching regions), there is an error in the frequency characteristic and design based upon prediction of such error is a troublesome task. To make the influence of the excitation and receipt between the electrode fingers 6 and 8 and between the electrode fingers 7 and 9 negligible, the tips of the intervening electrode fingers 7 and 11 and 8 and 12 should be disposed more closely to each other to reduce the area of the regions defined by the cross hatching lines. This approach, however, would increase the risk that the two fingers 7 and 11 and 8 and 12 would be short-circuited at their tip portions during formation of the electrode pattern. Because of different lengths of the respective electrode fingers in the electrode sets, it is not possible to provide a surface acoustic wave filter which manifests a non-symmetrical frequency response, using electrodes of the single finger type.
The latter method or the reflection method or the mirror method is fully discussed, for example, in ELECTRONICS LETTERS 28th Nov. 18, 1974, Vol. 10 No. 24 "Synthesis of Surface-Acoustic-Wave-Filters Using Double Electrodes" and U.S. Pat. No. 3,968,461. Briefly described, the mirror method assumes a virtual image of a central frequency 3f.sub.0 which is symmetrical with respect to 2f.sub.0, wherein f.sub.0 is the central frequency of a desired frequency characteristic. The resulting impulse response characteristic is similar to that available through the odd/even function method. The electrode pattern may also be similar to those as indicated in FIGS. 7A and 7B. This, however, suggests that the mirror method involves the same problems as with the surface acoustic wave filters as shown in FIGS. 7A and 7B.
A surface acoustic wave device disclosed in U.S. Pat. No. 3,968,461 which attains a non-symmetrical frequency characteristic is fabricated as follows. A plurality of interdigital electrodes are disposed at an interval equal to half of the wavelength of the surface acoustic waves corresponding to a frequency 2f.sub.0, at least some of which are arranged in sets with the two electrodes in each set are electrically connected in common. At least some of those electrode sets are held in different overlapping state from the respective adjacent electrode sets such that the frequency response characteristics of the interdigital electrodes centering at the frequencies f.sub.0 and 3f.sub.0 as a whole correspond to a Hermitian function and individually correspond to a non-Hermitian function. Because of different lengths of the respective electrode fingers in the electrode sets, it is not possible to provide a surface acoustic wave filter which manifests a non-symmetrical frequency response, using electrodes of the single finger type.